This invention relates to an irradiation process for making graft copolymers of an olefin polymer material.
Polyolefin graft copolymers can be made by any one of various methods, including forming active sites on the polyolefin either in the presence of the grafting monomers, or followed by treatment with the monomer. The grafting sites can be produced by treatment with a peroxide or other chemical compound that is a free radical polymerization initiator, or by irradiation with high energy ionizing radiation. The free radicals produced in the olefin polymer as a result of the chemical or irradiation treatment act as initiators for the polymerization of the monomer, as well as active sites for grafting. For example, U.S. Pat. No. 5,411,994 discloses making polyolefin graft copolymers by irradiating olefin polymer particles and treating with a vinyl monomer in liquid form. A non-oxidizing environment is maintained throughout the process.
Various additives have been used to modify characteristics of graft copolymers such as the morphology of the polymer particles. For example, U.S. Pat. No. 5,916,974 discloses graft polymerizing in the presence of an organic peroxide and a polymerization rate modifier (PRM) to increase the polymerization induction time on the polymer surface, consequently facilitating monomer diffusion into the interior of the polymer particles so that surface polymerization of the monomer is inhibited. Suitable PRMs include sulfur, benzoquinone and its derivatives, and hydroxylamine and its derivatives. The PRM has no significant impact on the number average and weight average molecular weight of the product.
There is a need for a process for controlling the molecular weight of the polymerized monomer side chains of polyolefin graft copolymers made from irradiated polyolefins so that low molecular weight side chains are produced, thereby improving the surface and internal morphology of the graft copolymers and improving processing without adversely affecting the overall physical properties of the graft copolymer.
The process of this invention for making graft copolymers comprises, in a substantially non-oxidizing atmosphere,
(1) irradiating a particulate olefin polymer material at a temperature of about 100 to about 85xc2x0 C. with high energy ionizing radiation to produce free radical sites on the olefin polymer material,
(2) treating the irradiated particulate olefin polymer material at a temperature of about 250 to about 90xc2x0 C. with about 0.5 to about 120 parts per hundred parts of the olefin polymer material of at least one grafting monomer that is capable of being polymerized by free radicals to form side chains on the olefin polymer material, in the presence of about 1 part to about 10,000 parts per million parts of monomer of at least one additive to control the molecular weight of the side chains of the polymerized grafting monomer, selected from the group consisting of (a) at least one hydroxylamine derivative polymerization inhibitor and (b) at least one chain transfer agent selected from the group consisting of (i) thio-substituted aliphatic and aromatic compounds, (ii) halogen-substituted aliphatic and aromatic compounds, (iii) nitro-substituted aliphatic and aromatic compounds, and (iv) aliphatic and aromatic phosphine derivatives, and
(3) simultaneously or successively in optional order, (i) deactivating substantially all residual free radicals in the resultant grafted particulate olefin polymer material, and (ii) removing any unreacted vinyl monomer from the material.
Carrying out the graft polymerization reaction in the presence of at least one hydroxylamine derivative polymerization inhibitor and/or one or more of the specified chain transfer agents produced graft copolymers with low molecular weight side chains. The graft copolymer product is easier to process and has improved internal and surface morphology.
The process of this invention for making graft copolymers comprises, in a substantially non-oxidizing atmosphere,
(1) irradiating a particulate olefin polymer material at a temperature of about 100 to about 85xc2x0 C. with high energy ionizing radiation to produce free radical sites on the olefin polymer material,
(2) treating the irradiated particulate olefin polymer material at a temperature of about 25xc2x0 C. to about 90xc2x0 C. with about 0.5 to about 120 parts per hundred parts of the olefin polymer material of at least one grafting monomer that is capable of being polymerized by free radicals to form side chains on the olefin polymer material, in the presence of about 1 part to about 10,000 parts per million parts of monomer of at least one additive to control the molecular weight of the side chains. of the polymerized grafting monomer, selected from the group consisting of (a) at least one hydroxylamine derivative polymerization inhibitor and (b) at least one chain transfer agent selected from the group consisting of (i) thio-substituted aliphatic and aromatic compounds, (ii) halogen-substituted aliphatic and aromatic compounds, (iii) nitro-substituted aliphatic and aromatic compounds, and (iv) aliphatic and aromatic phosphine derivatives, and
(3) simultaneously or successively in optional order, (i) deactivating substantially all residual free radicals in the resultant grafted particulate olefin polymer material, and (ii) removing any unreacted vinyl monomer from the material.
The propylene polymer material that is used as the backbone of the graft copolymer can be:
(1) a crystalline homopolymer of propylene having an isotactic index greater than 80, preferably about 85 to about 99;
(2) a crystalline copolymer of propylene and an olefin selected from the group consisting of ethylene and 4-10 C alpha-olefins, provided that when the olefin is ethylene, the maximum polymerized ethylene content is about 10%, preferably about 4%, and when the olefin is a 4-10 C alpha-olefin, the maximum polymerized content thereof is about 20% by weight, preferably about 16%, the copolymer having an isotactic index greater than 85;
(3) a crystalline terpolymer of propylene and two olefins selected from the group consisting of ethylene and 4-8 C alpha-olefins, provided that the maximum polymerized 4-8 C alpha-olefin content is 20% by weight, preferably about 16%, and, when ethylene is one of the olefins, the maximum polymerized ethylene content is 5% by weight, preferably about 4%, the terpolymer having an isotactic index greater than 85;
(4) an olefin polymer composition comprising:
(a) about 10% to about 60% by weight, preferably about 15% to about 55%, of a crystalline propylene homopolymer having an isotactic index greater than 80, preferably about 85 to about 98, or a crystalline copolymer of monomers selected from the group consisting of (i) propylene and ethylene, (ii) propylene, ethylene and a 4-8 C alpha-olefin, and (iii) propylene and a 4-8 C alpha-olefin, the copolymer having a polymerized propylene content of more than 85% by weight, preferably about 90% to about 99%, and an isotactic index greater than 85;
(b) about 5% to about 25% by weight, preferably about 5% to about 200%, of a copolymer of ethylene and propylene or a 4-8 C alpha-olefin that is insoluble in xylene at ambient temperature; and
(c) about 300% to about 70% by weight, preferably about 400/o to about 65%, of an elastomeric copolymer of monomers selected from the group consisting of (i) ethylene and propylene, (ii) ethylene, propylene, and a 4-8 C alpha-olefin, and (iii) ethylene and a 4-8 C alpha-olefin, the copolymer optionally containing about 0.5% to about 10% by weight of a polymerized diene and containing less than 70% by weight, preferably about 10% to about 60%, most preferably about 12% to about 55%, of polymerized ethylene and being soluble in xylene at ambient temperature and having an intrinsic viscosity, measured in decahydronaphthalene at 135xc2x0 C., of about 1.5 to about 4.0 dl/g,
wherein the total amount of (b) and (c), based on the total olefin polymer composition, is about 50% to about 90%, the weight ratio of (b)/(c) is less than 0.4, preferably 0.1 to 0.3, and the composition is prepared by polymerization in at least two stages and has a flexural modulus of less than 150 MPa;
(5) a thermoplastic olefin comprising:
(a) about 10% to about 60%, preferably about 20% to about 50%, of a propylene homopolymer having an isotactic index greater than 80, or a crystalline copolymer of monomers selected from the group consisting of (i) ethylene and propylene, (ii) ethylene, propylene and a 4-8 C alpha-olefin, and (iii) ethylene and a 4-8 C alpha-olefin, the copolymer having a polymerized propylene content greater than 85% and an isotactic index of greater than 85;
(b) about 20% to about 60%, preferably about 30% to about 500%, of an amorphous copolymer of monomers selected from the group consisting of (i) ethylene and propylene, (ii) ethylene, propylene, and a 4-8 C alpha-olefin, and (iii) ethylene and a 4-8 C alpha-olefin, the copolymer optionally containing about 0.5% to about 10% of a polymerized diene, and containing less than 70% polymerized ethylene and being soluble in xylene at ambient temperature; and
(c) about 3% to about 40%, preferably about 10/o to about 20%, of a copolymer of ethylene and propylene or a 4-8 C alpha-olefin that is insoluble in xylene at ambient temperature,
wherein the thermoplastic olefin has a flexural modulus of greater than 150 but less than 1200 MPa, preferably about 200 to about 1100 MPa; and most preferably about 200 to about 1000 MPa; or
(6) an ethylene homopolymer or a copolymer of ethylene and about 0.5% to about 35% of at least one 3-12 C alpha-olefin.
Room or ambient temperature is xcx9c25xc2x0 C.
The 4-8 C alpha-olefins useful in the preparation of (4) and (5) include, for example, butene-1, pentene-1; hexene-1; 4-methyl-1-pentene, and octene-1.
The diene, when present, is typically a butadiene; 1,4-hexadiene; 1,5-hexadiene, or ethylidenenorbornene.
Propylene polymer materials (4) and (5) can be prepared by polymerization in at least two stages, where in the first stage the propylene; propylene and ethylene; propylene and an alpha-olefin, or propylene, ethylene and an alpha-olefin are polymerized to form component (a) of (4) or (5), and in the following stages the mixtures of ethylene and propylene; ethylene and the alpha-olefin, or ethylene, propylene and the alpha-olefin, and optionally a diene, are polymerized to form components (b) and (c) of (4) or (5).
The polymerization can be conducted in liquid phase, gas phase, or liquid-gas phase using separate reactors, all of which can be done either by batch or continuously. For example, it is possible to carry out the polymerization of component (a) using liquid propylene as a diluent, and the polymerization of components (b) and (c) in gas phase, without intermediate stages except for the partial degassing of the propylene. All gas phase is the preferred method.
The preparation of propylene polymer material (4) is described in more detail in U.S. Pat. Nos. 5,212,246 and 5,409,992, which are incorporated herein by reference. The preparation of propylene polymer material (5) is described in more detail in U.S. Pat. Nos. 5,302,454 and 5,409,992, which are incorporated herein by reference.
The ethylene polymer used as olefin polymer material (6) can be an ethylene homopolymer or a copolymer of ethylene and about 0.5% to about 35% of at least one 3-12 C alpha-olefin. The copolymer can be, for example, linear low density polyethylene, but is not limited to this type of copolymer. The density of the ethylene polymer will be determined by the end use for which the graft copolymer is intended.
Propylene homopolymer is the preferred olefin polymer backbone material.
Suitable particulate forms of the olefin polymer material used in the present method include powder, flake, granulate, spherical, and cubic. When the monomer add level is high, i.e., greater than 20 parts of monomer per hundred parts of the olefin polymer material, it is preferable for some applications to use spherical particulate forms having a weight average diameter of about 0.4-7 mm, a surface area of at least 0.1 m2/g, and a pore volume fraction of at least about 0.07, and wherein more than 40% of the pores in the particle, preferably more than 50%, and most preferably more than 90%, have a diameter greater than 1 micron. The pore volume fraction is preferably at least 0.12, most preferably at least 0.20.
The olefin polymer material used as the backbone of the graft copolymer is irradiated with high energy ionizing radiation at a dose rate of about 1 to 1xc3x97104 megarads (Mrad) per minute for a period of time sufficient for the formation of free radical intermediates to occur, but insufficient to cause gelation of the polymer. The ionizing radiation can be of any kind, but the most practical kinds comprise electrons and gamma rays. Preferred are electrons beamed from an electron generator having an accelerating potential of 500-4000 kilovolts. Satisfactory results in terms of graft level are achieved with an ionizing radiation dose of about 0.5-12 Mrad, preferably about 2 to about 4 Mrad. The temperature during the irradiation step is preferably between about 10xc2x0 to about 85xc2x0 C.
The term xe2x80x9cradxe2x80x9d is usually defined as that quantity of ionizing radiation that results in the absorption of 100 ergs of energy per gram of irradiated material, regardless of the source of radiation. In the usual practice of the method described herein, energy absorption from ionizing radiation is measured by-the well know conventional dosimeter, a measuring device in which a polymer film containing a radiation-sensitive dye is the energy absorption sensing means. Therefore the term xe2x80x9cradxe2x80x9d means that quantity of ionizing radiation resulting in the absorption of the equivalent of 100 ergs of energy per gram of the polymer film of a dosimeter placed at the surface of the particulate olefin polymer material being irradiated.
The irradiated particles of olefin polymer material, while being maintained in a substantially non-oxidizing atmosphere (see below), are treated with at least one vinyl monomer as a liquid or in solution, optionally diluted with a suitable diluent, preferably by adding the liquid monomer or monomer solution to, and/or dispensing it onto the particulate material at a controlled rate, while the material is agitated or conveyed by any suitable means. Most preferably the liquid monomer or monomer solution is added by releasing a fine mist or spray of at least one monomer onto the irradiated particulate material while the particles are in motion, either relative to each other or to the point from which the monomer is released or dispensed. The temperature during the graft polymerization step is generally about 250 to about 90xc2x0 C., preferably about 25xc2x0 to about 50xc2x0 C., and most preferably about 350 to about 50xc2x0 C.
Solvents and diluents useful in the practice of the method of this invention are those compounds that are inert with respect to the particulate olefin polymer material and are not polymerizable by free radicals, and that have a chain transfer constant of less.than about 1xc3x9710xe2x88x923. Suitable solvents and diluents include ketones, such as acetone; alcohols, such as methanol; aromatic hydrocarbons, such as benzene and xylene; and cycloaliphatic hydrocarbons, such as cyclohexane.
The expression xe2x80x9csubstantially non-oxidizingxe2x80x9d is used to describe the environment or atmosphere to which the irradiated olefin polymer material is exposed before the deactivation of residual free radicals. The active oxygen concentration, i.e., the concentration of oxygen in a form that will react with the free radicals in the irradiated material, is less than about 15%, preferably less than about 5%, and more preferably less than about 1%, by volume. The most preferred concentration of active oxygen is 0.004% or lower by volume. Within these limits, the non-oxidizing atmosphere can be any gas, or mixture of gases, that is oxidatively inert toward the free radicals in the olefin polymer material, e.g., nitrogen, argon, helium, and carbon dioxide.
The grafting monomers that are capable of being polymerized by free radicals include any monomeric vinyl compound capable of being polymerized by free radicals, wherein the vinyl radical H2Cxe2x95x90CRxe2x80x94, in which Rxe2x95x90H or methyl, is attached to a straight or branched aliphatic chain or to a substituted or unsubstituted aromatic, heterocyclic, or alicyclic ring in a mono- or polycyclic compound. Typical substituent groups can be alkyl, hydroxyalkyl, aryl, and halo. Usually the vinyl monomer will be a member of one of the following classes: (1) vinyl-substituted aromatic, heterocyclic, or alicyclic compounds, including styrene, vinylnaphthalene, vinylpyridine, vinylpyrrolidone, vinylcarbazole, and homologs thereof, e.g., alpha- and para-methylstyrene, methylchlorostyrene, p-tert-butylstyrene, methylvinylpyridine, and ethylvinylpyridine, and (2) unsaturated aliphatic nitriles and carboxylic acids and their esters including acrylonitrile; methacrylonitrile; acrylic acid; acrylate esters such as the methyl, ethyl, hydroxyethyl, 2-ethylhexyl, and butyl acrylate esters; methacrylic acid; methacrylate esters, such as the methyl, ethyl, butyl, benzyl, phenylethyl, phenoxyethyl, epoxypropyl, and hydroxypropyl methacrylate esters. Multiple monomers from the same or different classes can be employed. When a hydroxylamine compound is used as the molecular weight control additive, monomers containing carboxylic acid groups should not be used because they will react with the hydroxylamine compound.
The total amount of monomer or monomers used is about 0.5 parts to about 120 parts per hundred parts. of the olefin polymer material. The preferred amount depends upon which monomer is used and upon the pore volume fraction of the polyolefin backbone.
During the graft polymerization, the monomers also polymerize to form a certain amount of free or ungrafted polymer or copolymer. Any reference to xe2x80x9cpolymerized monomersxe2x80x9d in this specification is meant to include both grafted and ungrafted polymerized monomers. The morphology of the graft copolymer is such that the olefin polymer material is the continuous or matrix phase, and the polymerized monomers, both grafted and ungrafted, are a dispersed phase. Although the weight average molecular weight of the grafted side chains of polymerized monomer cannot be measured directly, the weight average molecular weight (Mw) of the grafted side chains is correlated with the Mw of the chains of ungrafted polymerized monomer, since the polymerization conditions are similar in both cases.
Preparation of graft copolymers by contacting a liquid vinyl monomer with an olefin polymer material that has been irradiated with high energy ionizing radiation is described in more detail in U.S. Pat. No. 5,411,994, which is incorporated herein by reference.
The graft polymerization reaction of this invention is carried out in the presence of at least one additive that controls the molecular weight of the polymerized monomer, i e., one that produces low Mw side chains on the olefin polymer backbone. Low Mw in this regard means lower than the Mw of the polymerized monomer when the graft copolymer is made in the absence of a molecular weight control additive. The additive is present in an amount of about 1 part to about 10,000 parts per million parts of monomer, preferably about 100 parts to about 5000 parts, and most preferably about 250 parts to about 1500 parts.
The molecular weight control additive can be at least one hydroxylamine derivative polymerization inhibitor (PI) such as, for example, N,N-diethylhydroxylamine; N,N-dimethylhydroxylamine; N,N-dipropylhydroxylamine, and N-nitrosophenylhydroxyl amine. N,N-diethylhydroxylamine is preferred. More than one PI can-be used, provided that the compounds selected do not react with each other.
The molecular weight control additive can also be at least one chain transfer agent that is a thio-, nitro-, or halogen-substituted aliphatic or aromatic compound, or an aliphatic or aromatic phosphine derivative. Suitable chain transfer agents include, for example, octadecanethiol; bromotrichloromethane; triethylene glycol dimercaptan; benzene sulfide; dodecanethiol; mesityl disulfide; benzenethiol; hydrogen sulfide; carbon tetrabromide; carbon tetrachloride; tribromoacetic acid; 2,4,6-trinitroaniline; 2,4,6-trinitroanisole; 1,3,5-trinitrobenzene; phenyl phosphine, and diethyl phosphine. More than one chain transfer agent can be used, provided that the compounds selected do not react with each other. A combination of polymerization inhibitors and chain transfer agents can also be used, provided that the compounds selected do not react with each other.
When a polymerization inhibitor is used as the molecular weight control additive, the grafting efficiency is typically equal to or greater than the grafting efficiency of the control without an additive. When a chain transfer agent is used, the grafting efficiency is generally lower than that of the control without an additive. The desired grafting efficiency is determined by the end use of the product.
The graft copolymers of this invention can be formed into useful articles having improved surface and internal morphology. The surface of extrudates and films formed from the graft copolymers of this invention with low Mw side chains is much smoother than the surface of products made from graft copolymers with high Mw side chains. The low Mw side chains also make it easier to process, i.e., homogenize, the graft copolymers of this invention, and improve the internal morphology of the graft copolymer. When viewed in cross-section, there are finer domains of grafted and ungrafted polymerized monomer that are more uniformly dispersed in the olefin polymer matrix, which provides more uniform physical properties in the finished product.
Forming of the graft copolymers can be carried out by methods known in the art including, for example, thermoforming, injection molding, sheet extrusion, profile extrusion, and blow molding. Films and fibers can also be made from these graft copolymers. The graft copolymers of this invention can also be used as compatibilizers for olefin polymers as well as blends of olefin polymers and non-olefin polymers, and as coupling agents for glass-reinforced and mineral-filled polyolefins.
Isotactic index is defined as the percent of olefin polymer insoluble in xylene. The weight percent of olefin polymer soluble in xylene at room temperature is determined by dissolving 2.5 g of the polymer in 250 ml of xylene at room temperature in a vessel equipped with a stirrer, that is heated at 135xc2x0 C. with agitation for 20 minutes. The solution is cooled to 25xc2x0 C. while continuing the agitation, and then left to stand without agitation for 30 minutes so that the solids can settle. The solids are filtered with filter paper, the remaining solution is evaporated by treating it with a nitrogen stream, and the solid residue is vacuum dried at 80xc2x0 C. until a constant weight is reached. The percent by weight of polymer insoluble in xylene at room temperature is the isotactic index of the polymer. The value obtained in this manner corresponds substantially to the. isotactic index determined via extraction with boiling n-heptane, which by definition constitutes the isotactic index of the polymer.
Intrinsic viscosity is measured in decahydronaphthalene at 135xc2x0 C.
The melt flow rate (MFR) of the graft copolymers was measured using ASTM D-1238 at 230xc2x0 C. and 2.16 kg.
The pore volume fraction values were determined by a mercury porosimetry technique in which the volume of mercury absorbed by the particles is measured. The volume of mercury absorbed corresponds to the volume of the pores. This method is described in Winslow, N. M. and Shapiro, J. J., xe2x80x9cAn Instrument for the Measurement of Pore-Size Distribution by Mercury Penetration,xe2x80x9d ASTM Bull., TP 49, 3944 (Feb. 1959), and Rootare, H. M., xe2x80x9cA Review of Mercury Porosimetry,xe2x80x9d 225-252 (In Hirshhom, J. S. and Roll, K. H., Eds., Advanced Experimental Techniques in Powder Metallurgy, Plenum Press, New York, 1970).
The surface area measurements were made by the B.E.T. method as described in JACS 60, 309 (1938).
Weight average molecular weight was determined by gel permeation chromatography.
The grafting efficiency G=100xc3x97(Coxe2x88x92C)/Co, where C and Co are respectively the concentration (in pph of xylene) of the soluble polymerized monomer fraction and the total graft copolymer.
The percent conversion of both grafted and ungrafted monomer to polymer is equal to the weight of the total reactor product minus the weight of the propylene polymer starting material, divided by the weight of the starting monomer, and multiplied by 100.
In this specification, all parts and percentages are by weight unless otherwise noted.